HomeMy WebLinkAbout02 - Existing ConditionsCity of Sebastian Stormwater Master Plan Update
2 Existing Conditions
2.1 Geology and Soils
According to the Natural Resources
Conservation Service (NRCS) Soils Report
(1987),' a copy of which is included as
Appendix E, Indian River County is in the
coastal lowlands of the Atlantic Coastal Plain
physiographic province, which are the
remnants of ancient marine terraces. As
shown in Figure 2-1, the county includes four
distinct physiographic subdivisions, the Atlantic
Coastal Ridge, the Eastern Valley, the Ten
Mile Ridge, and the Osceola Plain.
The City of Sebastian is located in
northeastern Indian River County, between the
Indian River Lagoon and the South Prong of
the St. Sebastian River. The easternmost
portion of the City along the Indian River
Lagoon lies within the Atlantic Coastal Ridge,
where the soils are flat to gently sloping,
sandy, and well drained. The majority of the Figure 2-1. Geology and Physiographic Features of Indian River
City area lies west of the Atlantic Coastal County (from NRCS)
Ridge, in the Eastern Valley, where the soils
are predominately nearly level and wet. The topography of the City is very flat, with average grades of
approximately 0.1 percent.
o,nw. ft.
A shallow aquifer underlies all of the county and extends to a depth of approximately 150 feet below ground
surface, where it is underlain by the Hawthorne Formation. The Hawthorne Formation acts as a confining layer
and prevents the upward movement of water from the Floridan Aquifer, but also prevents the downward
movement of water from the shallow aquifer, which results in high groundwater elevations and reduced
percolation rates in the surficial soils. This is consistent with previous reports and observations from consultants
and City staff, which indicated that the soils are virtually impermeable over most of the City and open channels
have low infiltration rates and long recovery times.
Geologic and hydrogeologic information was obtained from the Soil Survey of Indian River County, published in
1987 by the U.S. Department of Agriculture's Natural Resources Conservation Service (NRCS). Geospatial soils
data was obtained via the NRCS Web Soil Survey and the download noted that the data was Version 19 dated June
8, 2020. The Soil Survey classifies soils into four distinct Hydrologic Soil Groups (HSG), A-D, depending on their
infiltration and runoff characteristics. HSG "A" soils, typically sands, have the lowest runoff potential while HSG "D"
soils like clays and wetland soils have the highest. Soils with two HSG groups (i.e., A/D) indicate that a confining
layer or high water table affects the infiltration rate that a given soil group would normally exhibit.
5 Soil Survey of Indian River County, Florida. United States Department of Agricultural, Soil conservation Service. January 1987
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City of Sebastian Stormwater Master Plan Update
The types, extents, drainage class and HSG of the soils within the City of Sebastian are shown below in Table 2-1.
The soils within the City of Sebastian grouped based on drainage class are provided in Table 2-2. The majority of
the City soils are poorly drained.
Table 2-1. City of Sebastian USDA/NRCS Soil Types
%.nuuee ♦gamy nne sans, irequenuy Punueu, u ru 170 swpes
u.3u
very puony urauieu
EauGallie fine sand
23.81
Poorly drained
Immokalee fine sand
18.04
Poorly drained
Myakka-Myakka, wet, fine sands, 0 to 2% slopes
12.22
Poorly drained
Oldsmar fine sand
9.33
Poorly drained
Pepper sand
0.90
Poorly drained
Riviera fine sand, 0 to 2% slopes
1.91
Poorly drained
St. Lucie sand, 0 to 8% slopes
1.34
Excessively drained
Archbold sand, 0 to 5% slopes
3.34
Moderately well drained
Wabasso-Wabasso, wet, fine sand, 0 to 2% slopes
0.28
Poorly drained
Pineda-Pineda, wet, fine sand, 0 to 2% slopes
1.33
Poorly drained
Quartzipsamments, 0 to 5% slopes
0.43
Somewhat poorly drained
Pomello sand, 0 to 5% slopes
5.98
Somewhat poorly drained
Arents, 0 to 5% slopes
2.77
Somewhat poorly drained
Floridana sand, frequently ponded, 0 to 2% slopes
0.06
Very poorly drained
Immokalee-Urban land complex
0.20
Poorly drained
Astatula sand, 0 to 5% slopes
6.96
Excessively drained
Satellite fine sand, 0 to 2% slopes
0.46
Somewhat poorly drained
Malabar fine sand
1.25
Poorly drained
Myakka fine sand, frequently ponded, 0 to 1 % slopes
2.60
Very poorly drained
Holopaw fine sand, 0 to 2% slopes
1.57
Poorly drained
Electra sand, 0 to 5% slopes
0.08
Somewhat poorly drained
Pompano fine sand, 0 to 2% slopes
0.07
Poorly drained
Riviera fine sand, frequently ponded, 0 to 1 % slopes
0.35
Very poorly drained
Lokosee fine sand
0.41
Poorly drained
Notes: Four percent of the City of Sebastian drainage area is covered in water and not included in the soil type table.
Table 2-2. City of Sebastian Soil Drainage Classes
Excessively drained
8.3
Moderately well drained
3.3
Poorly drained
71.3
Somewhat poorly drained
9.7
Very poorly drained
3.3
A/D
B/D
A/D
A/D
D
A/D
A
A
B/D
A/D
A
A
A
C/D
A/D
A
A
A/D
A/D
A/D
A
A/D
A/D
A/D
As shown in the data above, the high groundwater table effectively limits the infiltration capacity of the soils, which
is consistent with the local geological conditions described previously. These conditions, combined with the
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City of Sebastian Stormwater Master Plan Update
relatively flat topography, make drainage very challenging and restrict the types of BMPs that can be employed in
the SWMS, especially those that improve water quality.
2.2 Environmental
Stormwater runoff from the City of Sebastian drains either directly to the Indian River Lagoon (for areas east of
the Atlantic Coastal Ridge) or to the South Prong of the St. Sebastian River (via multiple outfalls) which ultimately
discharges to the Indian River Lagoon. The Indian River Lagoon is a 156-mile-long estuary along Florida's east
coast with impaired portions directly adjacent to lands in Volusia, Brevard, Indian River, and St. Lucie counties.
The Clean Water Act requires that the surface waters of each state be classified according to designated uses, such
as drinking water, aquatic life, recreation, and agriculture. Florida has six classes, based on the degree of protection
required. Currently, most surface waters in Florida, including many of those in the Central Indian River Lagoon
(CIRL), are categorized as Class III waters, meaning they must be suitable for recreation and must support fish
consumption and the propagation and maintenance of a healthy, well-balanced population of fish and wildlife.
In 2009, the FDEP adopted revisions to the Verified List of Impaired Waters for the CIRL that identified several
estuarine segments as impaired for dissolved oxygen (DO) due to low DO concentrations and nutrients due to an
imbalance in flora and fauna because of decreases in seagrass distribution.
The FDEP reported that intense and extensive algal blooms in the IRL began in 2011 and have returned
periodically. Harmful algal blooms (HABs) cause shading that stresses seagrass in the IRL, adverse effects on
wildlife, and in some cases, detrimental effects on human health.'
2.3 Drainage System Characteristics
The City of Sebastian uses a combination of traditional stormwater infrastructure and Green Infrastructure (GI) to
collect and convey stormwater runoff. The City's stormwater management system (SWMS) consists of the following:
• More than 360 miles of grassed swales and ditches (i.e., Roadside swales, Front Yard swales, Side Yard
ditches, Back Yard ditches, etc.
• 1,293 culverts, totaling more than 126,000 linear feet of pipe, and other drainage structures
• 30 retention and detention ponds
• 9 nutrient separating baffle boxes
• 8.4 miles of canals that discharge at seven locations to the South Prong of the St. Sebastian River.
The SWMS has two primary purposes — (1) to drain surface water to prevent flooding within habitable structures
and on roadways, and (2) to reduce pollutant loading to the City's canals, the South Prong of the St. Sebastian
River, and ultimately the Indian River Lagoon (IRL). In order to satisfy both of these seemingly conflicting
purposes, the SWMS must carefully balance the quantity of flows needed for proper drainage and the quality of
the drained water to prevent excessive pollutant loading to receiving waters. GI components such as canals, wet
detention and retention ponds are used to add capacity to the existing storm sewer system and increase removal
of pollutants while creating additional green space benefits for the community.
' Central Indian River Lagoon Basin Management Action Plan, February 2021.
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City of Sebastian Stormwater Master Plan Update
2.3.1 Previous Completed Improvements
The City has completed several important capital improvement projects in the SWMS since 2006, which are shown
in Table 2-3, and have had positive effects on the quantity and quality of the stormwater discharged. Some of the
listed projects were not previously identified on a SWMP update but were added separately by the City.
Table 2-3. C/P Projects Completed Since 2006
Davis Street Drainage/Baffle Box
2006
98724-1
Periwinkle Detention Pond
2008
103638-1 & FDEP Permit
George Street Drainage
2009
18714-4
Collier Canal Dredging/Seawall
2009
104663-2 & FDEP Permit (Section 319 Grant Funds)
Potomac Street Baffle Box
2009
119623-1 & FDEP Permit
Schumann Park Imp./Drainage
2009
40952-2
Barber Sport Complex Drainage
2010
40775-4
City Stormwater Park Update
2012
No Permit
Quarter Round Review
2013
No Permit
Stormwater Master Plan Update
2013
No Permit
Coolidge Street Baffle Box
2014
135385-1
Presidential Street Drainage
2014
130339-1 (FDEP TMDL Grant Funds)
Water Monitoring/Testing
2014-2017
No Permit
Seawall Investigations
2014-2016
No Permit
Northern Area Ditch Cleaning
2015
No Permit
FDEP MS4 NPDES Update
2016
MS4 Permit
Tulip Detention Pond
2016
134274-1 & ACOE Permit FDEP TMDL/Section 319 Funds)
Working Waterfront Baffle Box
2016
145977-1 & FDEP Permit SJRWMD Grant and TMDL Grant)
Oyster Bag Pilot Project
2016
142124-1 & ACOE Permit
CavCorp Parking Drainage
2016
142058-2
Jefferson Street Drainage Repair
2017
FDOT US-1 R/W Permit
2.3.2 Previously Identified Drainage issues
After reviewing the previous studies and discussions with City staff, the following issues related to the City's
stormwater management system have been identified.
2.3.2.1 Topography
The topography of the City is very flat, and the slopes of the conveyance structures (i.e., swales, ditches, culverts
etc.) are in many cases too shallow for proper flow velocities, which limits their capacity. If stormwater runoff
enters the conveyance structure at a greater rate than the flow capacity allows, the structure will rapidly fill up and
flood over its banks onto adjacent areas.
In an experimental effort to improve the water conveyance in swales with shallow slopes, the City initiated the
Quarter Rounds program in 2006, which used plastic pipes cut into quarters and installed along the bottom of the
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City of Sebastian Stormwater Master Plan Update
swale. The plastic pipe is a lower friction coefficient than grass, and field testing showed it did improve the
velocity. After installations were completed, the City found that homeowners were not adequately maintaining the
swale bottoms, so the City hired contractors at a cost of approximately $250,000 per year to clean swales
equipped with quarter round pipe. Maintenance costs continued to increase and the City ultimately determined
the program was not cost effective, so it was cancelled by the City Council in 2017.
2.3.2.2 Surface Water Elevations (Tailwater Controls)
The overall system is, for the most part, a tailwater controlled system. The conveyance paths end at a waterbody
(i.e., canal, St. Sebastian River, etc.), and if the water elevation within that downstream waterbody increases then
a tailwater control occurs, decreasing the drainage flow and causing upstream water elevations to increase. It
may not be possible or practical to solve all drainage problems within the City's system without reductions in
tailwater elevations.
Drainage issues related to tailwater controls will be further exacerbated by the future effects of climate change
and projected increases in sea level elevations in certain parts of the City. This issue is explained in more detail
in Section 5 — Sea Level Rise Assessment.
2.3.2.3 Groundwater Elevations
Groundwater elevations throughout most of the City are between 0-1 foot below the ground surface, which
significantly reduces or prevents the infiltration of stormwater from the swales, ditches, ponds, and canals and
makes water quality treatment efforts extremely challenging.
2.3.2.4 Geology and Soils
The soils and underlying geology in the Sebastian area create many challenges for the management of stormwater
as mentioned in Section 2.1. One of the most difficult challenges is the high groundwater conditions and
corresponding low infiltration rates in swales, combined with very shallow grades. This results in standing water in
many swales throughout the City after storm events. During system evaluations the addition of concrete linings in
swales was considered to improve the drainage velocities and flows, but such modifications would be costly, would
add substantial impervious area, and would increase runoff volumes that would likely overload downstream drainage
areas and not be acceptable to regulators. For these reasons, they were not selected as a CIP project.
2.3.2.5 Inadequate Stormwater Infrastructure
Many of the localized flooding and other drainage issues identified in previous studies and observed in the City are
the result of specific deficiencies in the stormwater management system. These deficiencies were further studied,
and corrective measures were recommended to the City in the form of Capital Improvement Program (CIP) projects.
Several of the identified CIP projects have been completed, but many others have not been addressed, presumably
because of a lack of funding. The updated CIP project inventory, including project descriptions, current status, and
estimated costs are included in Section 7 - Alternatives Development and CIP Prioritization.
2.3.2.6 Operations and Maintenance Difficulties
Maintenance of Open Channels - The presence of heavy vegetation reduces the flow velocity within an open
channel (i.e., grassed swale, etc.), which in turn reduces its flow capacity.
• Front Yard Swale — Per City Code Section 54-2-7.15(d), it is the continuous responsibility of any owner of
a lot to maintain such swales, ditches, and pipes on the site to maintain the proper flow of surface water.
Maintenance issues with front yard swales and driveway culverts are widespread and have been a
historical problem area in the management of the SWMS. The City has historically used code
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City of Sebastian Stormwater Master Plan Update
enforcement to correct non-compliance issues, but only when reported by residents or noticed by City
staff during other work activities. As noted above the inspection and maintenance of the swale is the
responsibility of the homeowner therefore the City does not currently conduct regular inspections of the
front yard swales.
In addition, the City mows the front yard swale of vacant lots to maintain adequate drainage, as the lot
owners historically have been unresponsive. On vacant lot parcels that have "Quarter Round" installed, the
City crews must hand clean the Front Yard swale to prevent damage to the PVC, which is labor-intensive
and costly to the City.
• Side Yard and Back Yard Ditches — previous issues related to the side yard and back yard ditches
included overgrown vegetation and unstable side slopes. The City began mowing these ditches through
a contract vendor several years ago on a quarterly basis, which has improved the vegetation issue.
However, the costs have increased rapidly and are a significant burden on the Stormwater Department's
finances. Also, as the elevations required for building foundations and septic tank mounds have increased
over the years, the slope of the side yard ditches also increased, rendering many of them unstable and
subject to rapid erosion.
• Aquatic Weed Control — the City currently contracts for spraying aquatic weeds with herbicides to control
them within the canals and minimize the discharge of live aquatic weeds downstream. This is costly to
the City but must be regularly performed to effectively control aquatic weeds within the City's canals.
• Canal dredging — Sediment buildup and shoaling was observed in the City's canals, indicating the need
for dredging to restore canal storage and conveyance capacity. City staff reported that the City has not
performed maintenance dredging on the canals since they were constructed, except for the Collier Canal
North, which was dredged during the 2009 bulkhead reconstruction project.
Canal bulkheads — There are currently 756 bulkheads along the banks of the City's canals. The City of
Sebastian owns and maintains approximately 20 seawalls within the City ROW along the banks of the
City's canals. The remainder of the 756 bulkheads are privately owned and maintained. The bulkheads
are constructed of several different materials, and more than 200 are structurally failing. For more
information on the condition of the bulkheads, see Section 3.3.3.1.
Maintenance of Retention/Detention Ponds
The City's SWMS includes 19 retention ponds and 11 detention ponds. The Stormwater Department
maintains one (Schumann Dr.), all others are maintained by the Parks Department. Sediment buildup
may have reduced the capacity of the ponds, they are not on a regular dredging schedule.
2.3.2.7 Environmental Concerns
As mentioned before, the St. Johns River Water Management District (SJRWMD) and South Florida Water
Management District (SFWMD) previously identified several drainage concerns for the St. Sebastian River
watershed. The principal concerns involve the seagrass coverage in the IRL and the volume of stormwater (fresh
water) discharged to the river and its dilution effects on the salinity within the receiving waters. Excessive
freshwater drainage into the Indian River Lagoon may result in species shifts that could have significant effects on
its fragile ecosystem. In addition, increased suspended matter and excessive loading of pollutants and nutrients
associated with stormwater runoff may further degrade water quality.
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